Prebake the pie crust until golden and fully set, then let it cool completely.
In a saucepan, whisk sugar, cornstarch, salt, water, lemon juice, and lemon zest, then cook until thickened.
Temper egg yolks with some hot mixture, then return to the pan and cook until smooth and thick.
Stir in butter, then pour the hot lemon filling into the cooled crust.
Beat egg whites with cream of tartar until foamy, then slowly add sugar and whip to stiff peaks.
Spread meringue over hot filling, sealing edges to the crust.
Bake until meringue is lightly golden.
Chill for at least 4 hours before serving.
